This invention relates to position sensing in fluid-operable actuators using pressure tappings and is particularly concerned with an improvement in or modification of the pressure tapping arrangement for sensing the position of a piston in a cylinder, the subject of our British Pat. No. 1,350,006. That specification is concerned with the provision of pressure tappings, and in particular co-operating pairs of pressure tappings, in a fluid-operable actuator including a relatively movable piston and cylinder. When the piston passes a position in the cylinder corresponding to a particular pressure tapping, there will be a change in the pressure at the tapping and so the pressure at the tappings is sensed to determine piston position. By using co-operating pairs of pressure tappings it is possible to determine piston position by sensing the pressure differential, if any, between the pressures at a pair of pressure tappings. Thus, when the piston moves to a position between a pair of pressure tappings, there is a pressure differential between them, whereas when the piston is at a position to one side of both the pressure tappings the pressure is approximately the same at both pressure tappings.
It is now appreciated that an improved response to position changes of the piston may be achieved by using a particular arrangement of pressure tappings.